Recently, in the use for air conditioning for vehicles, etc., in order to decrease influence to global warmth, a refrigeration cycle using a natural-system refrigerant such as carbon dioxide has been developed. However, with respect to carbon dioxide, in the use employed in a relatively high-temperature region such as an air conditioning system for vehicles, because the operational temperature in the refrigeration cycle exceeds the critical point of the refrigerant, it operates in a supercritical region in which the high-temperature-side refrigerant gas cannot be condensed. Since the temperature and the pressure of the gas in the supercritical region do not correspond to each other by a relationship of one to one, if there is a great fluctuation in the rotational speed or a great fluctuation in the load of the compressor driven by an engine in a vehicle and the like, the increase of the high-pressure side pressure is great, and an inconvenience, that the pressure exceeds an acceptable high pressure limit, is liable to occur. Therefore, a stable operation at a high pressure, such as an operation in a system where the temperature and the pressure of the refrigerant meet with each other by a relationship of one to one because of being condensed even at a high pressure as in a case using R134a and the like as the refrigerant, cannot be expected.
In order to solve this problem, Patent document 1 discloses the means wherein, in a variable displacement compressor controlling the displacement by a crank chamber pressure, a relief valve provided between a discharge chamber and a crank chamber is opened at the time of an abnormal high pressure, the crank chamber pressure is increased by flowing gas thereinto from the high-pressure side, and the displacement is decreased by decreasing the piston stroke of the compressor.
Further, Patent document 2 discloses the means wherein a suction pressure control means for controlling the displacement for discharge of the variable displacement compressor so that a predetermined suction pressure is achieved by a suction pressure detecting means and a discharge pressure control means for controlling the displacement for discharge so that a predetermined discharge pressure is achieved by a pressure detecting means for a discharge-pressure region are provided, and an abnormal increase of discharge pressure is suppressed by switching the control between the suction pressure control means and the discharge pressure control means depending on the detected pressure sent from the discharge pressure detecting means.
However, in the method for using a relief valve as described in Patent document 1, because the displacement of the compressor rapidly decreases when the discharge pressure exceeds a predetermined threshold value, the cooling operation of the refrigeration cycle is interrupted, and such a condition is not preferable from the viewpoints of comfortableness and cooling performance as a cooling device. Further, there is also a problem that the system and the device become complicated because a relief valve becomes necessary other than a control valve.
On the other hand, in the variable displacement compressor described in Patent document 2, because the suction pressure control and the discharge pressure control are employed by being switched, the control is improved from the viewpoints of comfortableness and cooling performance as a cooling device. However, there is a problem that the calculation routine of the control for switching the operations of the control valves, the detecting means and the control system become complicated. Further, an electronic control valve capable of externally controlling electronically is required as the control valve, and the structure and the cost as the whole of the system become complicated and expensive.
Patent document 1: JP-A-2002-61571
Patent document 2: JP-A-2005-127278